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Vischer Ferry Hydroelectric Project Underwater Repairs

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Every five years the New York Power Authority (NYPA) conducts an underwater inspection to assess the underwater conditions of their project at Vischer Ferry as a preventative measure to ensure the continued safe operation of the dam and facility. This includes observing undermining of the bedrock at the toe of the regulating structure, which has been monitored since 1995. The regulating structure is comprised of a series of gates, which are used together with the flow through the turbines to regulate the upstream water levels. Recent inspections revealed that the undermining condition was continuing to worsen. While the observed undermining was not judged to present an immediate threat to the dam, preventative action to provide continued safe operation of the facility was considered prudent.

NYPA engaged Gomez and Sullivan to design a remedy that would repair the current deterioration as well as reduce the potential of a future reoccurrence. During the bathymetric surveying for this work, a second large hole was discovered in the bedrock downstream of the apron for Dam F, which is perpendicular to the regulating structure.

Gomez and Sullivan completed a design which involved pouring tremie concrete at the base of Dam F and parallel to the regulating structure. Tremie concrete refers to a method of placing concrete underwater through a tube in a manner to enable continuous placement of the concrete without creating turbulence. This first step created a solid base whereupon a cofferdam was constructed, closing off the regulating structure toe to allow dewatering of the area. This allowed placement of concrete under the toe in a dry condition. BIDCO Marine Group, the contractor for the repairs, performed the first tremie pour comprising 140 cubic yards of concrete on September 5th, 2008.

On September 19th the second stage of repair was performed in the dry and involved the use of cast-in-place concrete to fill the hole below the regulating structure apron. An approximate total of 65 cubic yards of cast-in-place concrete was utilized to repair the hole and reinforce the surrounding surface to reduce/prevent future deterioration. The second hole below the Dam F apron was repaired underwater on September 19th as well, and required 110 cubic yards of tremie concrete to be satisfactorily filled.

All of the design and construction services were concluded by the end of September, and the Vischer Ferry project is currently back to normal operation.